Static random-access memory (SRAM) cell array and forming method thereof

ABSTRACT

A static random-access memory (SRAM) cell array forming method includes the following steps. A plurality of fin structures are formed on a substrate, wherein the fin structures include a plurality of active fins and a plurality of dummy fins, each PG (pass-gate) FinFET shares at least one of the active fins with a PD (pull-down) FinFET, and at least one dummy fin is disposed between the two active fins having two adjacent PU (pull-up) FinFETs thereover in a static random-access memory cell. At least a part of the dummy fins are removed. The present invention also provides a static random-access memory (SRAM) cell array formed by said method.

CROSS REFERENCE TO RELATED APPLICATIONS

This patent application is a continuation in part of and claims thebenefit of U.S. patent application Ser. No. 15/635,165, filed Jun. 27,2017, which is a divisional application of and claims priority to U.S.patent application Ser. No. 15/185,043, filed on Jun. 17, 2016, andentitled “STATIC RANDOM-ACCESS MEMORY (SRAM) CELL ARRAY AND FORMINGMETHOD THEREOF”, the entire contents of which are incorporated herein byreference.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates generally to a static random-access memory(SRAM) cell array and forming method thereof, and more specifically to aSRAM cell array applying dummy fins and forming method thereof.

2. Description of the Prior Art

Random access memory (RAM) can be operated to read data from it andwrite data into it. As the computers turn off, data disappearsimmediately. Since data in RAM can be altered easily, RAM is widely usedas temporary data storage memory in personal computers. RAM can beclassified into dynamic-type and static-type.

A dynamic random access memory (DRAM: Dynamic RAM) stores one-bit databy one transistor paired with one capacitor, and electricity must besupported during operating to keep this data, thus called Dynamic RAM.Dynamic RAM is a simple structure, therefore having slow access speedand costing low. Thus, it is often used as a memory providing highcapacity but low speed such as a main memory of a personal computer.

A static random access memory (SRAM: Static RAM) stores one-bit data bysix transistors, and electricity is not needed during operating to keepthis data, thus called Static RAM. Static RAM is a complex structure,therefore having high access speed and costing high, thereby it is oftenused as a memory providing low capacity but high speed such as a 256 KBor 512 KB cache memory built-in a central processing unit (CPU) of apersonal computer. Since a CPU mainly affects data calculating andprocessing speed of a computer while a main memory mainly affects datastorage capacity, a cache memory is utilized to save often-used data,thereby the CPU can more quickly reach this often-used data stored inthe cache memory, without reaching it in the main memory.

SUMMARY OF THE INVENTION

The present invention provides a static random-access memory (SRAM) cellarray and forming method thereof, which improves processing reliabilityand enhances performance of the static random-access memory cell array.

The present invention provides a method of forming a staticrandom-access memory (SRAM) cell array including the following steps. Aplurality of fin structures are formed on a substrate, wherein the finstructures include a plurality of active fins and a plurality of dummyfins, each pass-gate FinFET (PG FinFET) shares at least one of theactive fins with a corresponding pull-down FinFET (PDFinFET), at leastof the dummy fin is disposed between the two active fins having twoadjacent pull-up FinFETs thereover in a static random-access memorycell. The dummy fins are removed.

The present invention provides a static random-access memory (SRAM) cellarray including a plurality of fin structures located on a substrate.These fin structures include a plurality of active fins and a pluralityof remaining dummy fins (recesses) between the active fins andsurrounded by a plurality of isolation structures, wherein eachpass-gate FinFET (PG FinFET) shares at least one of the active fins witha corresponding pull-down FinFET (PD FinFET), and at least one of theremaining dummy fins (recesses) is disposed between the two active finshaving two adjacent pull-up FinFETs (PU FinFET) thereover in a staticrandom-access memory cell.

According to the above, the present invention provides a staticrandom-access memory (SRAM) cell array and forming method thereof, whichpatterns to form a plurality of fin structures on a substrate, whereinthe fin structures include a plurality of active fins and a plurality ofdummy fins, and then at least a portion of the dummy fins are removed.Thereby, gaps between these fin structures can be common orapproximately common through inserting the dummy fins between some ofthe active fins. Hence, profiles and shapes of these fin structures canbe common or approximately common, which improves device reliability andstability. At least one of the dummy fins is disposed between two of theactive fins of two adjacent pull-up FinFETs in a static random-accessmemory cell. Therefore, the two active fins of two adjacent pull-upFinFETs having larger gaps than the others can be adjusted to have thegaps approximate to the gaps of the others including gaps between finstructures in other areas such as logic areas.

These and other objectives of the present invention will no doubt becomeobvious to those of ordinary skill in the art after reading thefollowing detailed description of the preferred embodiment that isillustrated in the various figures and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically depicts a top view and a cross-sectional view of amethod of forming a static random-access memory (SRAM) cell arrayaccording to an embodiment of the present invention.

FIG. 2 schematically depicts a top view and a cross-sectional view of amethod of forming a static random-access memory (SRAM) cell arrayaccording to an embodiment of the present invention.

FIG. 3 schematically depicts a top view and a cross-sectional view of amethod of forming a static random-access memory (SRAM) cell arrayaccording to an embodiment of the present invention.

FIG. 4 schematically depicts a top view and a cross-sectional view of amethod of forming a static random-access memory (SRAM) cell arrayaccording to an embodiment of the present invention.

FIG. 5 schematically depicts a top view and a cross-sectional view of amethod of forming a static random-access memory (SRAM) cell arrayaccording to an embodiment of the present invention.

FIG. 6 schematically depicts a top view and a cross-sectional view of amethod of forming a static random-access memory (SRAM) cell arrayaccording to an embodiment of the present invention.

FIG. 7 schematically depicts a top view and a cross-sectional view of amethod of forming a static random-access memory (SRAM) cell arrayaccording to an embodiment of the present invention.

FIG. 8 schematically depicts a top view and a cross-sectional view of amethod of forming a static random-access memory (SRAM) cell arrayaccording to another embodiment of the present invention.

FIG. 9 is a cross-sectional view of a static random-access memory (SRAM)cell array using scanning electron microscope (SEM).

DETAILED DESCRIPTION

FIGS. 1-7 schematically depict top views and cross-sectional views of amethod of forming a static random-access memory (SRAM) cell arrayaccording to an embodiment of the present invention. Please refer toFIGS. 1-2, in which a plurality of fin structures 112 are formed on asubstrate 110. As shown in FIG. 1, a bulk bottom substrate 110′ isprovided. A hard mask layer 10 is formed on the bulk bottom substrate110′ and is patterned to define the location of the fin-shapedstructures 112, which will be formed in the bulk bottom substrate 110′.In this case, the hard mask layer 10 is an oxide layer 12 and a nitridelayer 14 stacked from bottom to top, but it is not limited thereto. Asshown in FIG. 2, an etching process P1 is performed to form thefin-shaped structures 112 in the bulk bottom substrate 110′. Thus, thefin-shaped structures 112 located on the substrate 110 are formedcompletely. In one embodiment, the hard mask layer 10 is removed afterthe fin-shaped structures 112 are formed, and a tri-gate MOSFET can beformed in the following processes. There are three contact faces betweenthe fin structures 112 and the following formed dielectric layerfunctioning as a carrier channel whose width is wider than a channelwidth in a conventional planar MOSFET. When a driving voltage isapplied, the tri-gate MOSFET produces a double on-current comparing tothe conventional planar MOSFET. In another embodiment, the hard masklayer 10 is preserved to form a fin field effect transistor (Fin FET),which is another kind of multi-gate MOSFET. Due to the hard mask layer10 being preserved in the fin field effect transistor, there are onlytwo contact faces between the fin structures 112 and the followingformed dielectric layer.

The present invention can also be applied to other semiconductorsubstrates. For example, a silicon-on-insulator substrate (not shown) isprovided, and then a single crystalline silicon layer being a top partof the silicon-on-insulator substrate (not shown) is etched till anoxide layer being a middle part of the silicon-on-insulator substrate(not shown) is exposed, meaning the fin-shaped structure formed on thesilicon-on-insulator substrate (not shown) is finished. For clarifyingthe present invention, fifteen fin-shaped structures 112 are depicted inthis embodiment, but the present invention can also be applied to aplurality of fin-shaped structures 112 with other numbers.

Please refer to FIGS. 3-6, the fin structures 112 are cut off to form alayout of a static random-access memory (SRAM) cell array. Methods ofcutting the fin structures 112 and forming the layout of the staticrandom-access memory (SRAM) cell array depend on processing and devicerequirements. In this case, the method of cutting the fin structures 112includes a first fin cut C1 and a second fin cut C2, wherein the firstfin cut C1 is depicted in FIGS. 3-4 and the second fin cut C2 isdepicted in FIGS. 5-6. Methods of forming the fin structures 112 mayinclude sidewall image transfer (SIT) methods. That is, the first fincut C1 or/and the second fin cut C2 may be steps included in thesidewall image transfer (SIT) methods. Therefore, the first fin cut C1or/and the second fin cut C2 may include cutting spacers used fortransferring patterns to the substrate 110 to form the fin structures112.

As shown in FIG. 3, a hard mask 20 is formed and patterned to cover aportion of the fin structures 112 not being removed, and expose theother portion of the fin structures 112 being removed. In this case, thehard mask 20 may include an organic dielectric layer (ODL) 22, asilicon-containing hard mask bottom anti-reflection coating (SHB) layer24 and a photoresist 26 from bottom to top. An edged fin 112 a and anedged fin 112 b are completely exposed and ends E of the fin structures112 between the edged fin 112 a and the edged fin 112 b are exposed bythe hard mask 20. Thereby, problems of fin structure connecting andline-end shortening occurring during applying a sidewall image transfer(SIT) method can be solved. Thereafter, the first fin cut C1 isperformed to completely remove the edged fin 112 a and the edged fin 112b, and the ends E of the fin structures 112 between the edged fin 112 aand the edged fin 112 b, as shown in FIG. 4, wherein the dashed-part isa cutting range of the first fin cut C1. After the first fin cut C1 isperformed, parts 112 a′/112 b′ formed from the edged fin 112 a and theedged fin 112 b may be preserved, and parts (not shown) formed from theends E of the fin structures 112 between the edged fin 112 a and theedged fin 112 b may also be preserved, wherein the parts 112 a′/112 b′protrude from the substrate 110 between the fin structures 112. Thefirst fin cut C1 may include cutting along many directions or onlycutting along one first direction. In this case, the first fin cut C1cuts along y-direction, and may optionally cut along x-direction toremove the edged fin 112 a and the edged fin 112 b, but it is notlimited thereto. In other cases, the first fin cut C1 may cut only alongy-direction, and thus the edged fin 112 a and the edged fin 112 b arepreserved. After the first fin cut C1 is performed, the photoresist 26,the silicon-containing hard mask bottom anti-reflection coating (SHB)layer 24 and the organic dielectric layer (ODL) 22 are removedimmediately.

A second fin cut C2 is performed. As shown in FIG. 5, a hard mask 30 isformed and patterned to cover a portion of the fin structures 112 notbeing removed, and expose the other portion of the fin structures 112being removed. In this case, the hard mask 30 may include an organicdielectric layer (ODL) 32, a silicon-containing hard mask bottomanti-reflection coating (SHB) layer 34 and a photoresist 36 from bottomto top. An edged fin 112 c and an edged fin 112 d are completely exposedby the hard mask 30. Thereafter, the second fin cut C2 is performed toremove the edged fin 112 c and the edged fin 112 d, as shown in FIG. 6,wherein the dashed-part is a cutting range of the second fin cut C2.After the second fin cut C2 is performed, parts 112 c′/112 d′ formedfrom the edged fin 112 c and the edged fin 112 d may be preserved,wherein the parts 112 c′/112 d′ protrude from the substrate 110 betweenthe fin structures 112. In this case, the second fin cut C2 cuts along asecond direction, meaning x-direction. Thus, the first direction of thefirst fin cut C1 is orthogonal to the second direction of the second fincut C2, but it is not limited thereto. After the second fin cut C2 isperformed, the photoresist 36, the silicon-containing hard mask bottomanti-reflection coating (SHB) layer 34 and the organic dielectric layer(ODL) 32 are removed immediately. In this embodiment, the hard masklayer 10 is immediately removed.

Two embodiments of forming a static random-access memory (SRAM) cellarray are presented in the following. FIG. 7 depicts a (1,1,1) typestatic random-access memory (SRAM) cell array, wherein each pass-gateFinFET (PG FinFET) shares one single active fin with a correspondingpull-down FinFET (PD FinFET). FIG. 8 depicts a (1,2,2) type staticrandom-access memory (SRAM) cell array, wherein each pass-gate FinFET(PG FinFET) shares two active fins with a corresponding pull-down FinFET(PD FinFET). Otherwise, the present invention can also be applied inother static random-access memory (SRAM) cell arrays, or other deviceshaving fin structures.

After the step of performing second fin cut C2 as shown in FIG. 6,portions of the fin structures 112 are removed to forma fin structurelayout used for transistors of a static random-access memory (SRAM) cellarray formed thereon, as shown in FIG. 7. As shown in FIG. 6, the finstructures 112 may include a plurality of active fins 112 e/112 f/112g/112 h/112 i/112 j and a plurality of dummy fins 112 k′/112 l′/112m′/112 n′/112 o′. A portion of the dummy fins 112 k′/112 l′/112 m′/112n′/112 o′ is removed to obtain a fin structure layout with one same finstructure shape. In this embodiment, five dummy fins 112 k/112 l/112m/112 n/112 o are formed after the dummy fins 112 k′/112 l′/112 m′/112n′/112 o′ are removed, wherein the dummy fins 112 k/112 l/112 m/112n/112 o protrude from the substrate 110 between the fin structures 112,as shown in the left diagram of FIG. 7, but it is not limited thereto.Thus, the layout of the active fins 112 e/112 f/112 g/112 h canconstitute a (1,1,1) type static random-access memory (SRAM) cell U1.The active fins 112 i/112 j are located in both sides of the (1,1,1)type static random-access memory (SRAM) cell U1. The active fins 112i/112 j may be active fins of other static random-access memory (SRAM)cells. The five dummy fins 112 k/112 l/112 m/112 n/112 o and the activefins 112 e/112 f/112 g/112 h/112 i/112 j are alternatively arranged. Inthis case, the dummy fins 112 k/112 l/112 m/112 n/112 o and the activefins 112 e/112 f/112 g/112 h/112 i/112 j are alternatively arrangedaccording to gaps of the active fins 112 e/112 f/112 g/112 h/112 i/112j, so that the fin structures 112 can have same gaps with each other andsame gaps as fin structures in other areas, but it is not limitedthereto. For example, gaps between active fins in a logic area may beless than gaps between active fins in a (1,1,1) type staticrandom-access memory (SRAM) cell U1 in current devices. Therefore, gapsbetween fin structures 112 can be equal to or approximately equal togaps between active fins in a logic area by alternatively inserting thedummy fins 112 k/112 l/112 m/112 n/112 o into the active fins 112 e/112f/112 g/112 h/112 i/112 j.

By applying methods of the present invention, that inserts at least adummy fin between active fins, gaps between fin structures of same areasor different areas can be common or approximately common, thereby thewidths, profiles and shapes of the fin structures can be common orapproximately common, and thus enhancing device performance such asreliability and stability. As the widths of the fin structures aredifferent, the performance of a formed static random-access memory(SRAM) degrades; as the shapes of the fin structures are different, theprocessing stability degrades. Furthermore, a maximum gap of the finstructures must be less than twice of a minimum gap of the finstructures (otherwise a dummy fin can be inserted into the maximum gap).In this embodiment, only a static random-access memory cell area A isdepicted and the static random-access memory (SRAM) cell U1 is disposedtherein, but the substrate 110 may also include a logic area, and gapsof the fin structures 112 in the static random-access memory cell area Aare preferably less than twice of gaps of the fin structures in thelogic area. Otherwise, as gaps between the fin structures 112 in thestatic random-access memory cell area A are larger than or equal totwice of gaps between fin structures in the logic area, at least a dummyfin can be inserted into gaps between the fin structures 112, so thatthe widths, shapes and profiles of the fin structures 112 in the staticrandom-access memory cell area U1 and the widths, shapes and profiles ofthe fin structures in the logic area can be the same or proximately thesame.

The (1,1,1)-type static random-access memory (SRAM) cell U1 includes twopull-up FinFETs PU1, two pass-gate FinFETs PG1 and two pull-down FinFETsPD1. Each pass-gate FinFET PG1 shares only one single active fin 112h/112 g with a corresponding pull-down FinFET PD1, and one single dummyfin 112 k is inserted between the two active fins 112 e/112 f with twoadjacent pull-up FinFETs PU1 disposed thereon. In a preferredembodiment, gaps P of the fin structures 112 are common. Likewise, thedummy fin 112 l/112 m is disposed between the shared active fins 112h/112 g and one of the active fins 112 f/112 e of the two adjacentpull-up FinFETs PU1 nearest to the shared active fins 112 h/112 g. Thedummy fins 112 o/112 h are respectively disposed between the sharedactive fins in two adjacent memory cells, meaning between the activefins 112 h/112 j and between the active fins 112 g/112 i.

It is emphasized that, the gaps P between fin structures 112 affect thewidths w and shapes of the fin structures 112. As the gaps P of the finstructures 112 are large, the slopes of cross-sectional profiles of thefin structures 112 are large, meaning the angle θ is larger; as the gapsP of the fin structures 112 are reduced, the slopes of cross-sectionalprofiles of the fin structures 112 are reduced, meaning the angle θ isreduced. Therefore, as the gaps between the fin structures 112 aredifferent, the widths and the slopes of cross-sectional profiles of thefin structures 112 are different. As the widths and the slopes ofcross-sectional profiles of the fin structures 112 are non-uniform, thereliability and stability of a formed device degrades. In thisembodiment, dummy fins 112 k/112 l/112 m/112 n/112 o are alternativelyinserted into the active fins 112 e/112 f/112 g/112 h/112 i/112 j toadjust gaps P between the fin structures 112, enabling gaps P betweenthe fin structures 112 and gaps P between the fin structures 112 and finstructures of other areas such as logic areas being the same. In thisembodiment, only one single dummy fin 112 k/112 l/112 m/112 n/112 o isinserted alternatively into the active fins 112 e/112 f/112 g/112 h/112i/112 j, but it is not limited thereto. The numbers of the dummy fins112 k/112 l/112 m/112 n/112 o may be optionally inserted into the activefins 112 e/112 f/112 g/112 h/112 i/112 j, or two or more than two dummyfins 112 k/112 l/112 m/112 n/112 o may be inserted into adjacent activefins 112 e/112 f/112 g/112 h/112 i/112 j, depending upon the gapsbetween the fin structures 112 and gaps between the fin structures 112and fins structures in different areas.

The (1,1,1)-type static random-access memory (SRAM) cell U1 may alsoinclude gate electrodes 120 over the fin structures 112, metalinterconnections 130 connecting each transistor including the pass-gateFinFETs PG1, the pull-down FinFETs PD1 and the pull-up FinFETs PU1, andthe contact plugs 140 physically connecting the gate electrodes 120 andthe metal interconnections 130. The structure of a (1,1,1)-type staticrandom-access memory (SRAM) cell and operating methods are known in theart and are not described herein.

The present invention may be applied in a (1,2,2)-type staticrandom-access memory (SRAM) cell array, as shown in FIG. 8. Thedifference between the (1,2,2)-type static random-access memory (SRAM)cell U2 and the (1,1,1)-type static random-access memory (SRAM) cell U1is that: the active fin 112 h is replaced with two active fins 112 h1/112 h 2, and a pass-gate FinFET PG2 in the (1,2,2)-type staticrandom-access memory (SRAM) cell U2 shares the two active fins 112 h1/112 h 2 with a corresponding pull-down FinFET PD2; the active fin 112g is replaced with two active fins 112 g 1/112 g 2, and the otherpass-gate FinFET PG2 in the (1,2,2)-type static random-access memory(SRAM) cell U2 shares the two active fins 112 g 1/112 g 2 with acorresponding pull-down FinFET PD2. The active fin 112 i beside the(1,1,1)-type static random-access memory (SRAM) cell U1 is replaced withtwo active fins 112 i 1/112 i 2, and the active fin 112 j beside the(1,1,1)-type static random-access memory (SRAM) cell U1 is replaced withtwo active fins 112 j 1/112 j 2. Still only one single dummy fin 112 kis disposed between the active fins 112 e/112 f of the adjacent pull-upFinFETs PU2.

Since a gap P1 between the two active fins 112 h 1/112 h 2, a gap P2between the two active fins 112 i 1/112 i 2 and a gap P3 between the twoactive fins 112 j 1/112 j 2 are less than gaps P4 between the otheractive fins, the dummy fins 112 k/112 l/112 m/112 n/112 o are disposedbetween active fins except for the two active fins 112 h 1/112 h 2, thetwo active fins 112 i 1/112 i 2 and the two active fins 112 j 1/112 j 2.Thereby, gaps between fin structures 112 can be adjusted to be as commonas possible. In this way, the widths and the shapes of the finstructures 112 can be the same, and thus improves processing reliabilityand enhancing performance of a formed static random-access memory (SRAM)array.

After the active fins 112 e/112 f/112 g(112 g 1/112 g 2)/112 h(112 h1/112 h 2)/112 i(112 i 1/112 i 2)/112 j(112 j 1/112 j 2) and the dummyfins 112 k/112 l/112 m/112 n/112 o are formed, isolation structures 40can be formed between the active fins 112 e/112 f/112 g(112 g 1/112 g2)/112 h(112 h 1/112 h 2)/112 i(112 i 1/112 i 2)/112 j(112 j 1/112 j 2),wherein the active fins 112 e/112 f/112 g(112 g 1/112 g 2)/112 h(112 h1/112 h 2)/112 i(112 i 1/112 i 2)/112 j (112 j 1/112 j 2) protrude fromthe isolation structures 40, but the isolation structures 40 blanketlycover the dummy fins 112 k/112 l/112 m/112 n/112 o.

Furthermore, FIGS. 7-8 show that the dummy fins 112 h/112 k/112 l/112m/112 n/112 o and the parts 112 a′/112 b′/112 c′/112 d′ protrude fromthe substrate 110. In other embodiments, the dummy fins 112 h/112 k/112l/112 m/112 n/112 o and the parts 112 a′/112 b′/112 c′/112 d′ may bereplaced by recesses R1/R2/R3/R4, as shown in FIG. 9. For example, whilethe dummy fins 112 h′/112 k 40/112 l′/112 m′/112 o′ of FIG. 6 areremoved, a part of the substrate 110 is also removed, and thus therecesses R1/R2/R3/R4 are formed. More precisely, a pull-down FinFET (PDFinFET) may be disposed over a fin structure 212, a pull-up FinFET (PUFinFET) may be disposed over a fin structure 214, and the other pull-upFinFET (PU FinFET) of a static random-access memory cell may be disposedover a fin structure 216. The recesses R1/R2/R3/R4 are formed in thesubstrate 110 beside the fin structure 212, the fin structure 214 andthe fin structure 216 respectively.

To summarize, the present invention provides a static random-accessmemory (SRAM) cell array and forming method thereof, which patterns asubstrate and forms a plurality of fin structures, wherein the finstructures include a plurality of active fins and a plurality of dummyfins, and then removes at least a portion of the dummy fins (or thedummy fins and a part of the substrate beneath the dummy fins). Thereby,gaps between fin structures can be common or approximately common byinserting dummy fins into active fins, so that the widths and shapes ofthe fin structures can be the same. Due to the widths and shapes of thefin structures of the present invention being the same, processingstability and device reliability can be improved.

More precisely, a formed static random-access memory (SRAM) cell arrayof the present invention may include two pull-up FinFETs, two pass-gateFinFETs and two pull-down FinFETs. Each pass-gate FinFET (PG FinFET)shares at least one active fin with a corresponding pull-down FinFET (PDFinFET). For instance, a (1,1,1)-type static random-access memory(SRAM)cell array of the present invention includes each of the pass-gateFinFETs (PG FinFET) sharing only one active fin structure with acorresponding pull-down FinFET (PD FinFET); a (1,2,2)-type staticrandom-access memory (SRAM)cell array of the present invention includeseach of the pass-gate FinFETs (PG FinFET) shares only two active finswith a corresponding pull-down FinFET (PD FinFET). It is emphasizedthat, at least one of the dummy fins (or recesses) is disposed betweenthe active fins of the two adjacent pull-up FinFETs in a staticrandom-access memory, thereby the two active fins of the two adjacentpull-up FinFETs currently having larger gaps than the other active finscan have gaps approximately common to gaps between the other active finsin the static random-access memory (SRAM) cell or gaps between finstructures in a same area or other areas such as a logic area. Byapplying the present invention, a maximum gap of the fin structures isless than twice of a minimum gap of the fin structures (otherwise adummy fin can be inserted into the maximum gap).

Those skilled in the art will readily observe that numerousmodifications and alterations of the device and method may be made whileretaining the teachings of the invention. Accordingly, the abovedisclosure should be construed as limited only by the metes and boundsof the appended claims.

What is claimed is:
 1. A static random-access memory (SRAM) cell array,comprising: a plurality of fin structures located on a substrate, thefin structures comprising a plurality of active fins, and a plurality ofrecesses between the active fins and surrounded by a plurality ofisolation structures, wherein each pass-gate FinFET (PG FinFET) sharesat least one of the active fin with a corresponding pull-down FinFET (PDFinFET), and at least one of the recesses is between the two active finshaving two adjacent pull-up FinFET (PU FinFET) thereover in a staticrandom-access memory cell.
 2. The static random-access memory (SRAM)cell array according to claim 1, wherein each of the staticrandom-access memory cells comprises two pull-up FinFETs, two pass-gateFinFETs and two pull-down FinFETs.
 3. The static random-access memory(SRAM) cell array according to claim 1, further comprising: at least oneof the recesses between the shared active fin and one of the active finsof the two adjacent pull-up FinFETs nearest to the shared active fin. 4.The static random-access memory (SRAM) cell array according to claim 1,further comprising: at least one of the recesses between the sharedactive fins in two adjacent static random-access memory cells.
 5. Thestatic random-access memory (SRAM) cell array according to claim 1,wherein the plurality of isolation structures are located between theactive fins and blanketly covering the recesses.
 6. The staticrandom-access memory (SRAM) cell array according to claim 1, wherein amaximum gap of the fin structures is less than twice of a minimum gap ofthe fin structures.
 7. The static random-access memory (SRAM) cell arrayaccording to claim 1, wherein the substrate comprises a staticrandom-access memory cell area having the static random-access memory(SRAM) cell array located therein, and a logic area, wherein gaps of thefin structures in the static random-access memory cell area are lessthan twice of gaps of the fin structures in the logic area.
 8. Thestatic random-access memory (SRAM) cell array according to claim 7,wherein the profiles of the fin structures in the static random-accessmemory cell area are common to the profiles of the fin structures in thelogic area.
 9. The static random-access memory (SRAM) cell arrayaccording to claim 1, wherein each of the pass-gate FinFETs (PG FinFET)shares only one active fin with a corresponding pull-down FinFET (PDFinFET).
 10. The static random-access memory (SRAM) cell array accordingto claim 1, wherein each of the pass-gate FinFETs (PG FinFET) sharesonly two active fins with a corresponding pull-down FinFET (PD FinFET).